java - Autowiring 定义为 in Spring 的集合

Question

我想拥有 Set<LocationResolver>像这样在 Spring 上下文中定义:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:context="http://www.springframework.org/schema/context"
       xmlns:util="http://www.springframework.org/schema/util"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-4.0.xsd
                           http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util-4.0.xsd
                           http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-4.0.xsd">

    <context:component-scan base-package="org.foo"/>

    <!-- Artifact LocationResolvers -->
    <bean id="fsLocationResolver" class="org.foo.storage.resolvers.FSLocationResolver" />
    <bean id="memoryLocationResolver" class="org.foo.storage.resolvers.InMemoryLocationResolver" />

    <!-- The Set<LocationResolver> -->
    <util:set id="resolvers" value-type="org.foo.storage.resolvers.LocationResolver">
        <ref bean="memoryLocationResolver"/>
        <ref bean="fsLocationResolver"/>
    </util:set>

    <bean id="configurationManager"
          class="org.foo.configuration.ConfigurationManager"
          init-method="init">
    </bean>

    <bean id="artifactResolutionService" class="org.foo.resolvers.ArtifactResolutionService">
        <property name="resolvers" ref="resolvers"/>
    </bean>

</beans>

我的 ArtifactResolutionService :

package org.foo.storage.resolvers;

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.Set;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;

@Component
public class ArtifactResolutionService
{

    @Resource(name = "resolvers")
    private Set<LocationResolver> resolvers;


    public Set<LocationResolver> getResolvers()
    {
        return resolvers;
    }

    public void setResolvers(Set<LocationResolver> resolvers)
    {
        this.resolvers = resolvers;
    }

}

我的 ConfigurationManager类:

package org.foo.configuration;

import org.foo.resource.ConfigurationResourceResolver;
import org.foo.storage.Storage;
import org.foo.storage.resolvers.ArtifactResolutionService;
import org.foo.storage.resolvers.LocationResolver;
import org.foo.xml.parsers.ConfigurationParser;

import java.io.IOException;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Scope;
import org.springframework.core.io.Resource;
import org.springframework.stereotype.Component;

@Component
@Scope ("singleton")
public class ConfigurationManager
{

    private static final Logger logger = LoggerFactory.getLogger(ConfigurationManager.class);

    private String configurationPath;

    private Configuration configuration;

    @Autowired
    private ConfigurationResourceResolver configurationResourceResolver;

    @Autowired
    private ArtifactResolutionService artifactResolutionService;


    public ConfigurationManager()
    {
    }

    public void init()
            throws IOException
    {
        Resource resource = configurationResourceResolver.getConfigurationResource("etc/configuration.xml",
                                                                                   "repository.config.xml",
                                                                                   "etc/configuration.xml");

        logger.info("Loading configuration from " + resource.toString() + "...");

        ConfigurationParser parser = new ConfigurationParser();

        configuration = parser.parse(resource.getInputStream());
        configuration.setResource(resource);

        dump();
    }

    public void dump()
    {
        System.out.println("Configuration version: " + configuration.getVersion());
        System.out.println("Listening on port: " + configuration.getPort());

        System.out.println("Loading storages...");
        for (String storageKey : configuration.getStorages().keySet())
        {
            System.out.println(" -> Storage: " + storageKey);
            Storage storage = configuration.getStorages().get(storageKey);
            for (String repositoryKey : storage.getRepositories().keySet())
            {
                System.out.println("    -> Repository: " + repositoryKey);
            }
        }

        // 
        // This throws a NullPointerException, as the resolvers are null.
        // 
        System.out.println("Loading resolvers...");
        for (LocationResolver resolver : artifactResolutionService.getResolvers())
        {
            System.out.println(" -> " + resolver.getClass());
        }
    }

    public void storeConfiguration(Configuration configuration, String file)
            throws IOException
    {
        ConfigurationParser parser = new ConfigurationParser();
        parser.store(configuration, file);
    }

    public Configuration getConfiguration()
    {
        return configuration;
    }

    public void setConfiguration(Configuration configuration)
    {
        this.configuration = configuration;
    }

    public String getConfigurationPath()
    {
        return configurationPath;
    }

    public void setConfigurationPath(String configurationPath)
    {
        this.configurationPath = configurationPath;
    }

    public ArtifactResolutionService getArtifactResolutionService()
    {
        return artifactResolutionService;
    }

    public void setArtifactResolutionService(ArtifactResolutionService artifactResolutionService)
    {
        this.artifactResolutionService = artifactResolutionService;
    }

}

resolvers , 然而，最终是 null我似乎无法弄清楚这一点。有什么明显的地方我做错了吗？

非常感谢!

Answer 1

有两件事值得警惕。

首先你有这个

<context:component-scan base-package="org.foo"/>

与

package org.foo.storage.resolvers;

@Component
public class ArtifactResolutionService

这意味着您正在隐式创建一个ArtifactResolutionService bean 。你还有

<bean id="artifactResolutionService" class="org.foo.resolvers.ArtifactResolutionService">
    <property name="resolvers" ref="resolvers"/>
</bean>

显式创建第二个 ArtifactResolutionService bean(bean 定义可能会相互覆盖，因为它们具有相同的 id)。在这两种情况下，您都有办法注入(inject) Set解析器。一个是 @Autowired , 另一个是 <property> .

其次，你的Spring配置不可能注入(inject)null在托管 bean 中。

您可能在 bean 的生命周期中尝试在错误的时间使用该字段，此时该字段尚未被注入(inject)。

看来你有循环依赖。 Spring 尝试注入(inject) ConfigurationManager进入 LocationResolver 之一在它 Autowiring 它的字段之前。为此，它需要调用 init-method .

---

注意带注解配置，我信这个

@Autowired
private Set<LocationResolver> resolvers;

实际上会拾取所有 LocationResolver bean ，不是你的Set bean 在这里定义

<!-- The Set<LocationResolver> -->
<util:set id="resolvers" value-type="org.foo.storage.resolvers.LocationResolver">
    <ref bean="memoryLocationResolver"/>
    <ref bean="fsLocationResolver"/>
</util:set>

如果你想要 Set bean，我会使用 @Resource

@Resource(name = "resolvers")
private Set<LocationResolver> resolvers;